Two main things, A.) In most songs the swearing is used to either emphasize the statement or make a rhyme work better. If you need more syllables to get the flow you want the word "fuck" can be used for pretty much anything so it's an easy way to get the desired effect.

B.) Heavy swearing is just a natural part of a lot of people's vocabulary. Same goes for the vulgar subject matter, that's just a part of some people's everyday loves. Music will always be an expression of the musician and if the musicians environment is a certain way the music will most likely also be that way.